1. Field of the Invention
The present invention relates to an improvement for a radiation detector to be used in a scintillation camera.
2. Description of the Related Art
If a radioactive medicine, which is labeled by radioisotope (hereinafter called "RI"), is added to a body, the radioactive medicine is absorbed in a specific organ or tissue, and concentrated thereon. By use of this property, a scintillation camera detects a .gamma.-ray, which is emitted from the radioactive medicine at random, for a fixed period of time, and obtains an RI distribution (scintigram). This is useful for the diagnosis of the shape and function of the organ, an existence of a focus, and a metabolic function.
A conventional radiation detector of an Anger type used in the scintillation camera is structured as shown in FIG. 1. More specifically, a scintilator 30 comprises an aluminum plate 32 and a glass plate 34, and a flange 35 of an iron frame. The aluminum plate 32 is attached to a front surface (.gamma.-ray incident surface) of a monocrystal 31. The monocrystal 31 is formed of sodium iodide (NaI) plate, and receives the .gamma. ray, and emits a flash. The aluminum plate 32 protects the monocrystal 31 from deliquescence. The glass plate 34 is adhered to the back surface (output surface of scintillation light) of the monocrystal 31 by a silicon adhesive 33. The glass plate 34 protects the monocrystal 31 from deliquescence, and scintillation light is passed therethrough. The scintilator 30 is annularly formed by a flange 35 of an iron frame not to be deformed.
The conventional radiation detector is structured as follows.
A plurality of studs 44 are formed in the flange 35, so that the frame is formed. A light guide 37 of an acrylic plate is adhered to the glass plate 34 of the scintilator 30 by hard two-part liquid silicon 36. A plurality of photomultipliers 39 having a photoelectric surface 40 on the bottom of the light guide 37 are tightly arranged on the upper surface of the light guide 37 by a coupling agent 38 such as optical grease. The photomultipliers 39 are pressed to the light guide 37 and fixed thereto by a case 43 through a spring 42, which is inserted into a socket 41. The scintillation light, which is generated in the monocrystal 31, is passed through the adhesive 33, glass plate 34, two-part liquid silicon 36, light guide 37, coupling agent 38, and the outer tube of the photomultiplier 39 in order, and detected at the photoelectric surface 40. In an electric circuit of the scintillation camera, an output of each photomultiplier 39 of the radiation detector is received. An energy signal in which the output is added, that is, a signal, which is proportional to the whole amount of emitting light of scintillation light, is selected by a window, which is set in accordance with energy of the incident .gamma. ray, so that only effective data is collected. Also, the output of the photomultiplier 39 is multiplied by a coefficient in accordance with the position of the arrangement, and added, so that the incident position of the .gamma. ray is calculated and the RI distribution is outputted.
The conventional radiation detector has the structure in which a plurality of members such as the glass plate 34, the light guide 37, and the like are layered. Due to this, when scintillation light exceeds a critical angle and enters the portion between the members having a different refractive index, for example, a boundary surface between the glass plate 34 and silicon 36 or a boundary surface between the silicon 36 and the light guide 37, scintillation light totally reflects. Therefore, the components of the total reflection is increased as the scintillation position is away from the center of sensibility of the photomultiplier 39. Due to this, the change of sensibility, which is called a positional response of the photomultiplier, to the position of scintillation of the photomultiplier 39 is decreased as the .gamma. ray enters the position, which is away from the center of sensibility. As a result, the electric circuit of the scintillation camera calculates the incident position of the .gamma. ray in a state that the incident position is biased to the center of sensibility, and shows a tendency for a luminescent spot to be concentrated on the center of sensibility. Therefore, there is a disadvantage in that uniformity, which is one of the important basic performance features of the scintillation camera, is decreased.
In the conventional scintillation camera, the output of the photomultiplier is nonlinearly compensated in the electric circuit based on the positional response in which a point source is collected upon being moved on the surface of the scintilator, thereby a decrease in uniformity is controlled.
However, the compensation circuit is excessively loaded by such compensation, and other basic performances of the scintillation camera such as intrinsic resolving power and energy resolving power must be given up to some extent.